The majority of mature T lymphocytes can be divided into two distinct phenotypes: CD8+ cytotoxic T lymphocytes (CTLs), which display the CD8 marker on their cell surface, and CD4+ helper T lymphocytes (T helper or TH cells), which display the CD4 marker on their cell surface. This subdivision is also associated with functional differences between the two cell types CTLs are, in general, involved in cell-mediated, or cellular, immune responses, and are activated by intracellular pathogens such as, for example, microbes and viruses. In particular, foreign antigens (e.g., viral antigens) are synthesized within infected cells and presented on the surfaces of such cells in association with class I major histocompatibility complex (MHC) molecules. CTL precursors display T cell receptors that recognize these antigens, triggering activation, maturation and proliferation of the precursor CTLs and resulting in CTL clones capable of destroying the cells exhibiting the antigens recognized as foreign.
T helper (TH) cells are involved in both humoral (i.e., antibody) and cell-mediated forms of immune response. With respect to the involvement of TH cells in humoral, or antibody, immune response, extracellular antigens are endocytosed by antigen presenting cells (APCs), processed and presented, preferentially in association with class II MHC molecules, to CD4+ class II MHC-restricted TH cells. These TH cells in turn activate B lymphocytes, resulting in antibody production. With respect to the role of TH cells in cell-mediated forms of immune response, some agents, such as mycobacteria which cause tuberculosis and leprosy, are engulfed by macrophages and processed in vacuoles containing proteolytic enzymes and other toxic substances. While these macrophage components are capable of killing and digesting most microbes, agents such as mycobacteria survive and multiply. However, the agents' antigens are processed by the macrophages and presented in association with class II MHC molecules to CD4+ class II MHC-restricted TH cells. These TH cells, in turn, become stimulated to secrete interferon-γ (IFN-γ) which activates macrophages. Such activation results in an increased bacteriocidal ability.
TH cells have been further categorized into two distinct subpopulations, termed TH1 and TH2 cell subpopulations. These two subpopulations of TH cells have been categorized on the basis of their restricted cytokine profiles and different functions. For example, TH1 cells are known to produce IL-2, tumor necrosis factor β (TNF-β) and IFN-γ. TH2 cells are known to produce interleukin 4 (IL-4), interleukin 5 (IL-5), interleukin 10 (IL-10) and interleukin 13 (IL-13). The different subpopulations are derived from a common precursor, or “naive” TH cell population (referred to as THP), and acquire their set pattern of cytokine production during a process referred to as “commitment.”
Genetic and environmental factors acting at the level of antigen presentation influence the commitment of a common naive T cell precursor to TH1 or TH2 differentiation. For example, the conditions of antigen stimulation (including both the nature and amount of antigen involved), the type of antigen-presenting cells and the type of hormone and cytokine molecules present all seem to represent determinants of the pattern of TH1 versus TH2 differentiation from a common naive T helper cell precursor. In particular, the decisive role appears to belong to the particular cytokines present in the cells environment. For example, IL-4, which is produced by TH2 and TH2-like cells, also appears to be an important factor in the commitment of naive THP cells to the TH2 subtype. Further, once TH1 and TH2 subpopulations are expanded, the two cell types tend to negatively regulate one another through the actions of cytokines unique to each subpopulation. For example, IFN-γ, which is produced by TH1 cells, negatively regulates TH2 cells, while TH2-produced IL-10 negatively regulates TH1.cells. Moreover, cytokines produced by TH1 and TH2 antagonize the effector functions of one another (Mosmann, T. R. and Moore, 1991, Immunol. Today 12:49). Although a full accounting of the exact factors important in driving TH1 and/or TH2 differentiation are, as yet, largely unknown, certain transcription factors activated in response to a given cytokine have been shown to be important in TH1 and/or TH2 differentiation. For example, the activation of signal transducer and activator of transcription (STAT)-6 by IL-4 has been shown to be important in TH2 differentiation, and the activation of STAT-4 has been shown to be important in TH1 differentiation (e.g., Romagnani, S., 1997, Immunology Today 18:263-266; Ray, A. and Cohn, L., 1999, The Journal of Clinical Investigation 104(8):985-993).
Although the TH1 and TH2 subtypes were originally identified in murine systems (see, for example, Mosmann, T. R. and Coffman, R. L., 1989, Ann. Rev. Immunol. 7:145), the existence of TH1-like and TH2-like subpopulations has also been established in humans (see, e.g., Del Prete, A. F. et al., 1991, J. Clin. Invest. 88:346; Wiernenga, E. A. et al., 1990, J. Imm. 144:4651; Yamamura, M. et al., 1991, Science 254:277; Robinson, D. et al., 1993, J. Allergy Clin. Imm. 92:313; Anderson, G. P. and Coyle, A. J., 1994, Trends in Pharmacological Sciences 15(9):324-32; Romagnani, S., 1997, Immunology Today 18:263-266). Human TH1-like and TH2-like cells have similar cytokine profiles to the TH1 and TH2 cells originally identified in murine systems, and preferentially express activation markers (e.g., CD30 and LAG-1). CD30, a member of the tumor necrosis factor (TNF) receptor family, is primarily expressed by TH2-like cells, and lymphocyte activation gene 3 (LAG-3) is preferentially expressed by TH1-like cells.
TH cells having characteristics (e.g., cytokine production profiles) of both TH1 and TH2 cell subpopulations have been designated TH0 cells (see, e.g., Firestein, G. S. et al., 1989, J. Imm. 143:518). CD8+ T cytotoxic (Tc)-cell subpopulations have also been identified based on the cytokines they produce. In general, activated CD8+ CTLs exhibit a TH1-like cytokine profile, but under some conditions CD8+ CTLs exhibit a TH2-like cytokine profile (Seder, R. A. et al., 1995, J. Exp. Med. 181:5-7; Manetti, R. et al., 1994, J. Exp. Med. 180:2407-2411; Maggi, E. et al., 1994, J. Exp. Med. 180:489-495). As noted above, TH1 and TH2 cell subpopulations appear to have great relevance to immune response against infectious agents such as viruses and intracellular parasites.
TH1-like and TH2-like cells appear to function as part of different effector functions of the immune system (see, e.g., Mosmann and Coffmann, supra). For example, TH1-like cells direct the development of cell-mediated immunity, triggering phagocyte mediated host defenses, and are associated with delayed hypersensitivity. Accordingly, infections with intracellular microbes tend to induce TH1-type responses. TH2-like cells drive humoral immune responses, which are associated with, for example, defenses against certain helminthic parasites and are involved in antibody and allergic responses.
Failure to control or resolve an infectious process often results not from an insufficient immune response but, rather, from an inappropriate response. Such inappropriate immune responses underlie a variety of distinct immunological disorders including, for example, mastocytosis (e.g., cutaneous mastocytosis and systemic mastocytosis), interstitial cystitis (IC), and atopic conditions (e.g., IgE-mediated allergic conditions) such as asthma, allergy (including allergic rhinitis), dermatitis (including psoriasis), systemic lupus erythematosus, scleroderma, pathogen susceptibilities, chronic inflammatory disease, organ-specific autoimmunity, graft rejection and graft versus host disease. For example, nonhealing forms of human and murine leishmaniasis result from strong but counterproductive TH2-like-dominated immune responses. Lepromatous leprosy also appears to feature a prevalent but inappropriate, TH2-like response.
Atopic conditions, such as asthma and allergy, are also examples of disorders that arise because of a TH2-like response to allergen (see, e.g., Holgate, S. T., 1997, Lancet 350 (suppl. II):5-9; Ray, A. and Cohn, L, supra; Oettgen, H. C. and Geha, R. S., 1999, The Journal of Clinical Investigation 104(7):829-835). In particular, such disorders are characterized by the development of IgE antibodies to foreign proteins. IgE antibodies are produced by B cells stimulated with IL-4, a cytokine produced by TH2 and TH2-like cells. Moreover, TH2-like cytokine profiles have been observed, not only in TH cells isolated from patients suffering from asthma and/or allergy, but also in mast cells and CD8+ CTLs isolated from such patients (Anderson and Coyle, supra). Further, animal studies have demonstrated that TH2-like cells play an important role in the induction of inflammation and the chronic pathological changes associated with asthma. For example, the constitutive expression of TH2 cytokines (e.g., IL-4 and IL-5) in mice has been shown to induce an asthma-like syndrome (Ray, A. and Cohn, L, supra).
A bias towards a TH2-like response has also been suggested to contribute to the loss of control of the immune system over HIV infection. In particular, a drop in the ratio of TH1-like cells to other TH cell subpopulations has been suggested to play a critical role in the progression toward disease symptoms. Further, it has been noted that, at least in vitro, TH2-like clones appear to be more efficient supporters of HIV viral replication than TH1-like clones (Romagnani, S., supra).
Further, while TH1-mediated inflammatory responses to many pathogenic microorganisms are beneficial, such responses to self antigens are usually deleterious. It has been suggested that the preferential activation of TH1-like responses is central to the pathogenesis of such human inflammatory autoimmune diseases as multiple sclerosis and insulin-dependent diabetes. For example, TH1-type cytokines predominate in the cerebrospinal fluid of patients with multiple sclerosis, pancreases of insulin-dependent diabetes patients, thyroid glands of Hashimoto's thyroiditis, and gut of Crohn's disease patients, suggesting that such patients mount a TH1-like, not a TH2-like, response to the antigen(s) involved in the etiopathogenesis of such disorders.
A primary goal, for both diagnostic and therapeutic reasons, therefore, would be the ability to identify, isolate and/or target members of a particular TH cell subpopulation. As such, the identification of genes which are differentially expressed within and/or among TH cell subpopulations is desirable. To date, investigations have focused on the expression of a limited number of specific known cytokines and cytokine receptors in the TH cell population. Cytokines, however, exert effects on cell types in addition to specific TH cell subpopulations, i.e., exhibit a variety of pleiotropic effects. It would be beneficial, therefore, to identify reliable markers (e.g., gene sequences) of TH cell subpopulations whose effects are TH cell subpopulation specific, e.g., which, unlike secreted cytokines, are TH cell subpopulation specific.
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